Reciprocal circuit for secret communication



June 4, 1946. A, V` BEDFORD E1- A| 2,401,405

RECPROCAL CIRCUIT'FOR SECRET COMMUNICATION Filed March 26, 1945 4 Sheets-Sheet 1 W we n 5 mw 1 mi wm @www f L www z, M Mmmfmw @a 2 u IYJL M H 5 jl v M uw www.. K mfg maw a w E, a.. Mm, z w M Hmm .a p Fna i@ mi. a ,21%, H ,VdK/r z f M M A l a 7 m m, f vl|||||l|s||||rl|||l|ll.. mi K aw; c l.) mmwf 9, m @ma MM m m J wm 7n @o ma f mw wm I H v W im s A. v. BEDFORD ETAL RECIPROCAL-CIRCUIT FOR SECRET COMMUNICATION Filed March 26,l 1943 Ivi/IER ffm/f I June 4, 1946. A. v. BEDFORD ErAL 2,401,405

RECIPROCAL CIRCUIT FOR SECRET COMMUNICATION Filed Mrch 26, 1943 4 sheets-sheet 3 Gttorneg June 4, 1946. A. v. BEDFORD Er AL RECIPRCCAI. CIRCUIT FCR SECRET COMMUNICATION Filed march 26, 1945 4 'sheets-sheet 4 w Al Patented .Fue 24 1946 2.4.01,06 y RECEPRDCAL GIRCJIT FOR SECRET 'arm CATION Alda V. Bedford and Frank P. Wipm'ceton,

N. J., assignore to Radio Corporation of America, a corporation oi Delaware Application March 26,'

l Our invention relates to the production of the reciprocal of a signal or wave and particularly to the production of such a signal for use in a system for secret communication.

In the copending application of Alda V. Bedford, Serial No. 456,578. filed August 29, 1942. and entitled Secret communication system, there is described a communication system in whichv the speech or other communication signal S is multiplied by a coding signal K and in which the l0 product SK is transmitted. At the receiver the 'incoming signal SK is multiplied by the reciprocal of the coding signal K to obtain the original signal S, that is, SK X 1/K=S. According to the above Bedford application the coding signal 15 K may be generated by means of a code disc which is cut to make its periphery have the wave `form of the signal K while the decoding signal l/K may be produced by a different disc which is cut in accordance with calculated values to make its periphery have the wave form of l/K.

An object of the present invention is to provide an improved method of and means for producing the reciprocal of a signal or wave.

A further object of the invention is to provide v an electrical circuit which will convert an applied signal having a certain wave form to a signal having a kwave form that is a reciprocal of the applied wave form.

A further object of the invention is to provide an improved communication system for secret signalling.

In practicing ou'r invention we supply a carrier wave e1 oi constant amplitude through a. mixer circuit to a main amplifier. A negative feed-back .35

amplifier is connected to feed signal from the output circuit of the main amplifier back to the mixer circuit where the signal e1 and the output oi the feed-back amplier-are added.' This added signal is lthe signal that is impressed upon the 40 input terminals of the main ampliiier. The coding signal K is applied to the feed-back amplifier to vary its gainin direct proportion to the amplitude of the coding signal K. It can be shown' mathematically that if the gain of the amplifiers is large, the output of 'the main amplifier, to a close approximation, is the reciprocal l/K times a constant. However. it should be noted that in the embodiment to be described the apparatus works only on the positive swing of a signal and,

therefore, two units` such as shown -in' Fig. 2 are required in the practical embodiment.

In utilizing our reciprocal circuit in a coin-` munication system, a coding signal K is generated in any suitable marmer at the transmitter, as by 1943, Serial No. 480,716

(-Cl. Uil- 1.5)

means of a-coding disc, the speech signal S is. multiplied by the signal K, and the signal SK is transmitted to the receiver. At the receiver there is another signal generating devicey for producing the signal K; it may be a coding disc .like that at the transmitter, thetwo coding discs belngrun in synchronism. The signal K generated at the receiver is supplied to the reciprocal circuit to` obtain the decoding 'signal l/K which is then multiplied with the received signal SK to obtain the original signals.

The invention will be better understood from the'following description taken in connection with the accompanying drawings in which Figure 1 is a block diagram of va communication system embodying our invention.

Figure 2 is a block diagram illustrating the principle oi' a reciprocal circuit embodying ourinvention,

Figure 3 is a circuit diagram of a reciprocal circuit embodying o invention,

Figures i to 12, inclusive. are graphs which are referred to in explaining the operation of the reciprocal circuit of Fis. 3.

4Figure 13 is a block diagram showing another embodiment of our invention.

Figure 14 is a graph thatv is' referred to in explaining Fig. 13, and

scribed. Amicrophone anda speech ampliiier are shown at 1 and 8, respectively. vThe signal S is applied through al switch 9 to a multiplier unit it which may be of the same design as that described in thev above-identiiled Bedford application. The code signal K may be produced by means oi' a code disc I6. a mask i'L'a light source IB. a condensing lens i9, and a photoelectric cell 20. The signal K is supplied through 'ampliners 2l and 22 and through a .switch to the multiplier unit I6.l The resulting multiplier output signal S'K is supplied through an ampliiier 26 vand through a switch 2li to a radio transmitter 28 or to a wire line. it preferred. A

The receiver for dec the signal SK may be the same apparatus as in Eis. 1 with -Aswitches s, zz and 21 thrown to their' oontaot positions .,R. The multiplier I4 is unchanged. The signal through the switch 21 to headphones .or to the loudspeaker I3. l

The code disc I8 at the transmitter and the corresponding code disc atv the receiver are held in synchronism and in the proper phase relation by suitable synchronizingmeans. For example, at the transmitter a GOO-cycle per second lcurrent from a source I6 may be supplied through a switch 31 to a synchronous motor n which rotates the code disc i8. The GOO-cycle cur-V rent also modulates a radio transmitter 39 for the transmission of synchronizing signal to the receiver. At the receiver the switch l1 is in its contact position R. whereby the received 600-cycle current is supplied from a radio receiver 4I to the synchronous motor 3l. In some cases it may be preferable to transmit the synchronizing signal over a wire line.

Fig. 2 shows in block diagram the basic part of the reciprocal circuit 32. It comprises an oscillator- 42 which supplies a carrier-frequency voltage e; of constant amplitude and frequency.

The voltage e1 passes through a mixer or voltage-adding circuit 43 toa main amplifier 44 having a gain G. The voltage output e: of ampliner 44 is fed back to the mixer 48 through a feedback amplifier 4l having a gain n which is varied in direct proportion to the coding signal K which is impressed thereon. The voltage output of amplier 4B is et, this voltage being-'added to the voltage e1 in the mixer 4l whereby the mixer output voltage is ea=e1+c4. f

It may be shown as follows that `the output' voltage es 1/K(Cz) where C: is a constant. In practice Cb represents a direct-current voltage which is blocked by coupling capacitors. The values e1, cz, es and e4 used in the following equa-- tions are the Vinstantaneous amplitudesv of the envelopes oi." the carrier wave and et, respectively.

Then i m, But 0G 1s large compared with Therefore lf g=KC1," where C', is a constant then ezgclK-L-Cland if e, is constant, y'

egg l/K(C,) where .C1 is a constant.

voltages e1, ez. es

4 broken line blocks 4I, 44, 44 and 4ta, 44a and 4ta, one group acting upon the positive portions of the K wave and the other group acting upon the negative portions of the K wave;

The signal -K is impressed upon an amplifier tube 5l which supplies the signal +K from its l anode circuit and the signal -K from its cathode circuit. The signal +K is supplied over a conductor 52 to a grid Il of the pentagrid tube 53. (comprising the feed-back amplitler 44) to control its gain. A suitable negative bias is supplied to the grid through a grid-leak resistor 55. `'I'he signal -K is supplied over a conductor 52a to a grid of the pentagrid tube lla Acomprising the feed-back amplifier 46a, to control its gain.

A carrier wave oscillator 42, such as a 200 kc. sine wave oscillator-supplies a carrier wave to Vthe mixer circuits 4l and 43a which comprise blocking capacitors l1, 58 and 51a, 54a, respectively, and resistors Il and 59a, respectively.

The output of mixer 43 feeds into the main ampliiler 44 which includes a pair of resistance coupled amplifier tubes 4I and 62 biased to operate as class A amplifiers. In the example illustrated, they are biased by means of cathode resistors 63 and 84 having bypass capacitors Il and 41. 'I'he output of the tube 42 is applied through a blocking capacitor 4I to a control grid B9 of the feed-back tube Il. A suitable ne!- ative bias is supplied to the grid 4l through a grid resistor 1|. The output of the feed-back tube Il is supplied over a conductor 12 to the niixerl circuit 43 where this output voltage adds tothe carrier wave voltage supplied from oscillator 42. As described'in connection with-Fig. 2,

' since the main ampliiler 44 and the feed-back amplifier 44 have high gains G and o, respectively, the instantaneous amplitude ofthe envelope The output M1 of ampliiier l44 is shown in Y Fig. 6. It will be Seen that it is modulated downward during the positive parts M o! the wave K shown in Fig. 4; during the negative parts N of the K wave, the carrier wave remains at full amplitude. This is a result of biasing the grid Il of' the feed-back tube 63 soV that the tube. Il

is driven to cut-off during the occurrence of the negative parts N of the wave K, there, of course,

being no negative feed-back while the tube is cut-off.

The output wave Mi is supplied through-a blocking capacitor 1l to a grid 14 of a pentagrid 'amplifier tube 11. A normal negative biss is applied tothe grid 1s through a grid-leak resistor 14. The .wave K is suppliedpfrom the anode circuit of the tube ll over a conductor 1I to the grid 1I for the vpurpose of making the tube 'I1 effective to'pass a signal only during the positive parts M of the wave K. During the negative parts N of the wave K. the grid 1l is driven negative sumciently to drive the tube 1 1 to anode current cutoiI: During the positive portions M of the wave K, the tube 11 is made operative to pass the s18- of large amplitude'to ensure proper control ci.'v 4

the tube 11. It will be noted that the wave vK 7 is supplied from the conductor 1l to a blockingv na1.M1 because the `portions M drive the grid 14 slightly positive so long as they are being applied to the grid. The wave K from conductor 1I is capacitor li and through a grid-leak resistor 42 in the input circuitof tube 11. The grid 'i8 is prevented from going more than slightly positive by means of a resistor 83 which has a large voltage drop produced therein byany substantial flow of grid current.

As shown in Fig. 8, the envelope of the carrier wave output M2 of the tube His proportional to the reciprocal of only the positive portions M of the coding wave K, there being no signal d'uring the negative portions N. The signal M2 is rectiiied by a diode 86 which has an output resistor 88 shunted by a bypass capacitor 81. vThe rectied signal Ma (Fig. 9) is supplied through a resistor 88 to the grid of an output tube 88.

The circuit and the operation of the other group of mixer and amplifier units 53a, Ma and 48a and of the amplifier 11a and diode 86a are the same as described above, the only difference being that in this second group of units it is the negative portions N of the wave K that determine the modulation of the carrier. It will be noted that the wave -K (Fig. is supplied from the cathode circuit of the tube 5I over the conductors 52a and 19a. The two 'groups of units have similar parts indicated by the same reference numerals with the letter a following the numerals applied in the second group of units.

ascisse The carrier wave output Ni (Fig. 7) of the main amplier 64a is modulated only by the portions N of the wave K. The unmodulated portions of the wave Ni are removed by the amplier tube 'ila to produce a wave N2 (Fig. 10) which corresponds to the wave Mz but which contains only the reciproeal of the negative portions N of the wave K. The output of the diode Bdais the signal N4 illustrated in Fig. 11.. The combined diode output signals Ma and N4 are the desired reciprocal signal l/K shown in Fig. 12. It will be noted that the diode 88a is reversed-with respect to the diode 8d in order to give the N portions .of signal K their original negative polarity.

If desired, the carrier wave signals N12 and N may be utiiizcd in the multiplier circuit is (Fig.

1) without rectification. `This is illustratedA in Fig. 13 which shows a multiplier circuit o f the type described in the above-identied Bedford nal as indicated in Fig. le. The negative portion The posi- B is reversed in polarity by a tube 98. tive portion A is multiplied in'multiplier tubes 96 and 9B by the reciprocal signals Mz and N2, respectively. One of the. resulting signals AM-.i is passed through a high pass illter El to remove the original -low frequency signal (primarily to remove signal S when the multiplierlis used for transmission) and is demodulated by a detector 98 to obtain an audio signal which is supplied to an adding circuit or mixer 99. The other signal AN'.` is passed through a high pass lter iti, a detector 602 and a reversing tube it@ to obtain a second audio signal which is supplied to the mixer 99. Similarly, the signal portion B is multiplied in multiplier tubes IM and ill by the reciprocal sig- 'nais M2 and Na, respectively. Thesignal BM: is c passed through a high pass illter lill, a detector cation signal.

w8 and a reversing tube iBS to supply a third audio signal to the mixer 99. The signal BN: is passed through a high-pass filter lil and a detector H2 to supply a fourth audio signal to the mixer SQ.

The four audio signalsadd at the mixer 99 to give the desired signals. Any carrier frequency. l

components that may be present are filtered out byalowpassillter H8.

Fig. 15 -shows one possible multiplier tube arrangement that may be employed for the multiplier units 9S, 93, IM and IGS of Fig. 13. It consists of a three-element vacuum tube, for example, that is biased to operate on a curved portion of its characteristic. This bias is varied by the audio signal, such as signal A, so that a carrier wave signal, M2 for instance, applied to the grid of the tube is modulated by the audio signal.

This modulation also is a multiplication of the two signals whereby the output, in the example assumed, is AMs. It is evident that a tube operating as described as a multiplier is capable of multiplying only voltages which are swung in a positive direction with respect tothe A. C. axis of the wave.

From the foregoing description it will be seen that by' means of our invention it is possible to obtain the reciprocal of any electrical signal regardless oi its wave form merely by passing the signal through an electrical circuit. This method of obtaining a reciprocal wave is much superior to the method comprising calculating and cutting a reciprocal wave form on a disc, both because it saves time and labor, and because it cdas nexibllity to the design of a signallingsystem. It should be understood that our method and means for obtaining reciprocal values are not limited to use in a secret signalling system, but are of more general application.

We claim as our invention:

1. The method of communication which comprises producing a communication signal and a distorting signal, multiplying said signals to obtain their product, transmitting said product to a receiver, producing at the receiver a second distorting signal which is substantially a duplicate of the first distorting signal,- producing substantially the reciprocal of the second distorting signal by passing it through an electrical circuit, and multiplying the received product signal by said reciprocal signal to obtain the original communi- 2. The method of communication which comv prises producing a communication signal and ya distorting signal, multiplying said signals to obtain their product, transmitting said product to a receiver, producing at the receiver a secondriistorting signal which is substantially a duplicate of .the iirst distorting signal, converting the second distorting signal to a signal that is substantially a reciprocal oi' the second distorting signal, and multiplying the received product signaly by' said reciprocal signal to obtain the original communicationsignal.

3. A communication system which comprises means for producing a communication signal, means for producing a distorting signal, means for multiplying said signals to obtain their product. means for transmitting' said product to .a

receiver, means for producing at the. receiver a second distorting signal which is substantially a duplicate of the first distorting signal, means at the receiver for producing from the second distoi'tiig Signal a signal that is substantially the received product signal by said reciprocal signal to obtain the original communication signal.

4. A communication system which comprises means for producing a communication signal, means for producing a distorting signal, means for multiplying saidsignals to obtain their product, means for transmitting said product to a receiver, means for producing at the receiver .a second distorting signal which is substantially a duplicate of the first distorting signal, means comprising an electrical circuit 'through which said second distorting signal is passed for converting it to a signal that is substantially the reciprocal thereof, and means for multiplying the received product signal by said reciprocal signal to obtain the original communication'signal.

5. The invention according to claim 4 wherein means is provided for )maintaining between the communication signal and the distorting signal substantially the phase relation at the receiver as at the transmitter.

6. In a vcircuit comprising a main amplifier, a negative feed-back amplifier, and an adding circuit, the method of obtaining the reciprocal of an electrical signal which comprises producing a carrier wave signal and passing it through said adding circuit, amplifying the output of the adding circuit in said main amplifier, feeding back the amplified signal through said negative feedback amplifier to said adding circuit, and varya negative feed-back amplifier having its input ing the gain of the 4feed-baci: amplifier substantially in direct proportion to said electrical signal whereby its reciprocal appears in the output of the main amplifier.

'7. An electrical circuit for obtaining the reciprocal of a signal which comprises a main ampliiler, a negative feed-back amplifier having its input circuit connected to the output circuit of the main. amplifier and having its output circuit connected through a voltage adding circuit to the y.input circuit of the main' amplifier, means for supplying a carrier wave signal to said adding circuit, and means for varying the gain of the feed-back amplifier in direct proportion to the amplitude of said first signal whereby its reciprocal appears in the output of the main amplifier.

8. In a circuit for obtaining the reciprocal of a signal, two main amplifiers, each main amplifier having a negative'feed-back circuit that in'. cludes a feed-back amplifier, the output circuit of each feed-back amplifier being .connected through a signal adding circuit to the input circuit of the associated main amplifier, means for supplying a carrier wave signal to said signal adding circuits, and means for varying the gain circuit connected to the output circuit of the main amplier and having its output circuit connected through a voltage adding circuit to the input circuit of the main amplifier, means for suppli/ins a' carrier wave signal to said adding-circuit, means for varying the gain of the feedback amplifier indirect proportion tdithe amplitude of the positive portions of said` first sisnal whereby the reciprocal of said positive portions appears in the output of the main amplifier, a second main amplier, a second negative feed-back amplifier having its input circuit connected to' the output circuit of the second main amplifier and having its output circuit connected through a second voltage adding circuit to the input circuit of the second main amplifier, means for supplying a carrier wave signal to said second adding circuit, and means for varying the gain of the second feed-back amplifier in direct proportion tothe amplitude of the negative portions of said rst signal whereby the reciprocal of said negative portions appears in the output of the second main amplifier, and means for adding said reciprocals of said two portionsto 'obtain the reciprocal of said first signal. ,4 i i 12. An electrical circuit for obtaining the reciprocal of a signal having positive and negative portions, said circuitcomprising a main amplifier, a negative feed-.back amplifier having its input circuit connected to the output circuit of the main amplifier and having its output circuit connected through a voltage adding circuit to the input circuit of the maiinamplifler, meansfor supplying a carrier wave signal to said adding circuit, means for varyiniif.l the gain of the feed-back ampliiler in direct proportion to the amplitude of the positive portions of said first signal whereby the reciprocal of said positive portions appears in lthe output of the main amplifier, a second main amplifier, a second negative feed-back amplifier having its input circuit connected to the output circuit of the second main amplifier and having its output circuit connected through a second Y voltage adding circuit to the input circuit of the of the two feed-back amplifiers in direct proportion to the amplitudes of the positive and negative portions, respectively, of said first signal whereby the reciprocal values of said portions appear in the'output circuits of the mainam 9. 'I'he invention according to claim 8 wherein means is provided for supplying theoutputs ot said main amplifiers to two channels, respectively, each of which includes a signal blocking means, and means for unblocking said channels-only when signal portions having reciprocal 4values are beingsupplied thereto. v

10. The invention according to claim 8 wherein means is provided for rectifying the outputs of said main amplifier and wherein means is provided for reversing the poiarity'of one of the rectified outputs and for adding it to the other rectified output. i

second main amplifier, means for supplying a carrier wave signal to said second adding circuit,l

and means for varying the gain of the second feed-back amplifier in direct proportion to the amplitude of the negative portions of said first signal whereby the reciprocal of said negative portions appears in the output of the second main amplifier, means for rectifying the outputs of said main amplifiers, means for reversing the polarity of one of said rectified outputs vand for adding it to the other rectified output to obtain the recip- I rocal of said first signal. 1

13. Wave shaping means for reshaping an electrical wave, said means comprising an electrical network that converts' said electrical wave to a second electrical wave having a wave form that is substantially the. reciprocal of the wave form ofthenrstwsve. i Y

14. In combination, means for producingan electrical coding or signal distorting-wave havins s.v certain wave form, and electrical network means for converting said wave to a second electrical wave` having a wave form that is substanz tially the reciprocal of thewave form of the first WQNQ- ALDA v. BEDFORD mm1; P. wrpr'r. 

